usb: dwc3-msm: remove global context variable

Now that all uses of the global context variable have been removed,
remove the variable itself. This allows the dwc3-msm driver to
support multiple device instances.

Change-Id: Ic1aa6b6a16323cc03870101a157ec35de0c6899b
Signed-off-by: Jack Pham <jackp@codeaurora.org>